Automated machinery, such as robots, are used in the modern industry in ever-increasing number. In order to prevent accidents in such automated installations, it is therefore important to have a reliable system to detect the presence of humans near the moving parts of machines. During normal operation, robots work in an autonomous manner. However, the presence of humans near the robots is essential during maintenance and programming operations. In order to prevent injuries to technicians due to collision with the robot arm, there is need for a detection system which will produce a signal when the arm comes dangerously close to a person. This signal can be used to stop the movement of the robot arm.
A number of different approaches have been used in designing detection systems of human presence, such as:
a) Ultrasonic system PA1 b) Infrared system PA1 c) Microwave Doppler radar system PA1 d) Photoelectric system PA1 e) "Sensitive skin" tactile system PA1 f) System using artificial vision PA1 g) Capacitive system
Research has been done on these detection methods and the most promising ones in industrial situations have been the vision and the capacitive systems. It is potentially very appealing to have cameras installed above and around a robot workcell, and this vision system can then be used to stop the robot arm when collision with a person becomes imminent. However, to operate in real time in this manner, the image has to be captured and analyzed in a few milliseconds. This is not feasible in the present state of technology. Better success has been achieved by simplifying the image, wherein sources of light are installed on the helmet of the technician and on the extremity of the robot arm. The vision system then monitors the distance between these two points and emits an alarm signal if this distance becomes smaller than a predetermined value.
In the capacitive systems, use is made of the fact that the capacitive coupling between two metal antennas, or between a single antenna and the ground, will be increased by the nearby presence of a human body or other conductive objects in the vicinity. A number of devices have been proposed, where the increase of capacitance caused by the approach of an object near the antenna is measured by a suitable method, such as the resonance conversion method or the reactance conversion method.
The prior art in this field may be found described in the following articles: a) MILLARD, D. L. (1989), "An In-Situ Evaluation of a Capacitive Sensor-Based Safety System for Automated Manufacturing Environments", Paper No. MS89-301, Society of Manufacturing Engineers, P.O. Box 930, Dearborn, Mich. 48121; and b) JACQUET, P., FAULHABER, J. P., DE KERMOYSAN, A., and KNEPPERT, M. (1987), "Protection proximetrique sur robot: Application d'un detecteur capacitif", Cahiers de notes documentaires no ND 1618-126-87, Institut National de Recherche et de Securite, 30, rue Olivier-Noyer, 75680 Paris cedex 14, France.
The prior art also may be found in patents. For example, U.S. Pat. No. 4,345,167 issued to Calvin on Aug. 17, 1982 discloses a method for detecting a human; the device measures the change in capacitance in an antenna as a person approaches. An antenna is repetitively charged with a voltage and discharged trough a discharge resistor and an averaging capacitor. The approach of the antenna by a person results in a larger discharge current, and hence, an increased voltage across the resistor. The voltage is AC coupled to a comparator, the change in state of which signals an approach.
U.S. Pat. No. 4,169,260 issued to Bayer on Sep. 25, 1979 discloses a capacitive intrusion detector utilizing a reference oscillation drift. Protected bodies are connected to form an antenna that is excited by a voltage controlled oscillator, the phase of which is compared to that of a reference oscillator signal. The presence of a person causes the capacitive reactance of the antenna and the frequency of the oscillator to change. This change in frequency is manifest as a shift in phase between the signal of the oscillator and that of the reference oscillator. The change in phase causes an output that indicates the presence of a person.
U.S. Pat. No. 4,661,797 issued Apr. 28, 1987 to Schmall describes capacitive electrodes fixed to the arm of a robot to determine the oscillation frequency of a measuring circuit. A variation in the impedance due to the proximity of a human body or even its direct contact therewith causes a frequency variation which is detected.
Although capacitive detection systems have been used in the recent past in various applications, there has been frequent cases of false triggering caused by variations of environmental factors (such as temperature and humidity of air) and interference from other systems in the plant, such as walkie-talkies. Another problem that plagues the use of capacitive systems in robotic installations is that a change of capacitance is produced not only by the presence of a human, but also because of the presence of many other types of materials (such as a piece of metal). The robot arm must be stopped if it comes too close to the body of a person. However, the detection system must not stop the arm when it approaches a piece of metal, because that is a part of the normal activity of the robot. This problem has been solved by recording an "ideal" signature pattern when the robot performs a typical work cycle. During the regular operation of the robot in production, the signature of each cycle is compared with the ideal signature. If the robot arm happens to approach a person, the two signatures will no longer match and the arm will be stopped. This system works satisfactorily well when the robot continuously repeats identical movements during automated production. However, no fixed pattern of arm movement is repeated during programming or maintenance operations, and this method can no longer be used to identify an unsafe situation which is produced when the robot arm moves very close to the body of the person.
Thus, none of the prior art devices identified above is capable of discriminating between a human and an inanimate object.